Method of piezoelectrically activating ferroelectric materials



D.19,1967 MASAO TAKAHASH. ETA. 3,359,470

METHOD OF PIEZOELECTRICALLY ACTIVATING FERROELECTRIC MATERIALS Filed Aug. l0, 1965 2 Sheets-Sheet l DAKS LAPSED FROM THERMAL OEPOLAR/ZAT/ON 70 PL/NG FIGLI nwe/waas. msm mmf/Asl# Fam/0 mMAc/CH/ Non/o rsa/@00cm @y 70,146.1/ ohwo ATTO NEYS.

Dec. 19, 1967 MASA@ TAKAHASHI ET AL 3,359,470

METHOD OF PIEZOELECTRICALLY ACTIVATING FERROELECTRIC MATERIALS Filed Aug. lO, 1965 2 Sheets-Sheet 2 RooM TEMR RooM TEMP. RaoM TEMR /00"c: ONE M/NuTE AFTER F/VE M//vurss AFTER TEN M/NUTEs AFTER TE/v MINUTES AFTER APPL/EA T/o/v oF APPL/CA T10/v 0F APPL/EA T/o/v 0F APPL/EA T/o/v aF A c voz TA EE A c voL TAGE A c VOL TAGE Ac VEL TAGE Proza F1621 Flea; Fused Pb gra 52 7'`0 45) 03 alu/'. 70Cr2 03 TME ELPSE AFTER THERMAL DEPOLRZT/ON: @NE DAY RoaM TEMR ROOM TEMP. Roo/w TEMA T00c ONE M//vuTE AFTER F/VE MWL/TEE AFTER TEN MTNA/TEE AFTER TEN M//vurEs AFTER APPL/cAT/a/v oF ARRL/EA T/o/v 0F lARRL/EA T/o/v 0F APPL/cAr/o/J 0F Ac vm TAGE Ac voLTAEE Ac Vm TAGE Ac VEL TAGE F 16.33 FIGBB FIGB c: FlGd .IA/VENTURE. Pb (2052 7 0-43)03*05t%6203 MAsAo TAKAHAsH/ TIME ELAPSE AFTER THERMAL 5gg/lg gU/Zg/ccl oERoLAR/ZAT/ON: THREE Hal/Rs MEW OHNO ,4 T TOR/VEYS.

United States Patent ABSTRACT OF THE DISCLOSURE A poling process for ferroelectric materials whereby an AC electric field greater than the coercive electric eld intensity and less than the breakdown eld intensity of the material is applied thereto, and thereafter a DC electric field is applied, to produce improved characteristics in the material.

This invention relates to poling processes for ferroelectric materials.

The invention, with all of the objects, features and advantages thereof will be best understood by reference to the description herein set forth taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a graph illustrating dependence of the electromechanical coupling factor in the radial mode (k1.) on the lapse of time of a ferroelectric material whose comp'osition can vbe expressed as:

Pb(Zr0,52Ti0,48)O3-l0.5 wt. percent Fe203, and produced by the poling process when the lapse of time from the thermal depolarization process to the poling process is varied.

FIGS. 2a, 2b and 2c show hysteresis curves as viewed on a cathode-ray tube screen, of several specimens at room temperature of a ferroelectric ceramic which can be expressed as:

Pb(Zr0.52Ti 48)O3-i0.1 wt. percent Cr203, by subjecting the specimen to a maximum A.C. voltage of kv./ mm. for time intervals of 1 minute, 5 minutes, and l0 minutes, respectively, after they have been left standing in air for 24 hours at the conclusion of thermal depolarization.

FIG. 2d shows a hysteresis curve produced on a cathode-ray tube screen when a specimen having the same composition was subjected to a maximum AC voltage of 5 kv./mm. at 100 C. for 10 minutes after a time elapse of 24 hours at the conclusion of thermal depolarization.

FIGS. 3a, 3b and 3c show hysteresis curves produced on a tube screen, of several specimens at room temperature of a ferroelectric material which can be expressed as:

Pb(Zr0 52Ti0 48)O3-I0.05 wt'. percent Ga203, when the specimens were subjected to a maximum AC voltage of 5 kv./mm. for time intervals of 1 minute, 5 minutes, and minutes, respectively, after being left standing in air for three hours at the conclusion of thermal depolarization, land FIG. 3d shows such ahysteresis curve when a specimen having the same composition was subjected to a maximum A.C. voltage of 5 kv./mm. at 100 C. after a time elapse of three hours at the conclusion of thermal depolarization.

As those knowledgeable in the art are aware, ferroelectric materials in general exhibit the characteristics of both ferroelectricity and piezoelectricity. Displacement type ferr'oelectric materials such as are represented by barium titanate (BaTiO3) and lead titanate zirconate Pb(Zr-Ti)03 manifest no piezoelectricity even if a pair of electrodes are installed on the opposite surfaces of a specimen regardless of whether they are ceramic materials produced by sintering in terms of solid phase reaction or single crystal materials produced by melting in terms of liquid phase reaction. The reason is due to the fact that the internal structure of such material is divided ferroelectrically into what may be called ferroelectric domains.

Stated more specifically, piezoelectricity possessed by one domain has a direct bearing on the direction of the spontaneous polarization axis and the spontaneous polarization of all domains spatially distributed at random within la displacement type ferroelectric material even if a pair of electrodes are simply installed without applyi-ng la high DC electric field. Therefore, if 'a domain is elongated in proportion t'o the intensity of an externally applied electric eld, another domain whose spontaneous polarization is in the opposite sense to that domain must be contracted by the same amount, with the result that no overall piezoelectricity is exhibited. In order that the piezoelectricity characteristic may be exhibited, the spontaneous polarization of these domains should be reoriented in one common direction. Furthermore, if the spontaneous polarization of all domains is reoriented in one direction, the greatest piezoelectric effect would be manifested by any given specimen.

A common means for effecting the reorientation is to apply a high intensity DC electric eld to a ferroelectric material for a considerably long time interval to reorient the direction of spontaneous polarization of each domain in what is commonly called the easy axial direction of each domain which is the closest to the direction of the applied DC electric iield. Such process is known as poling All piezoelectric materials, however, do not belong to the crystallographic cubic structure, and accordingly in certain materials a stress is created during reorient'ation. In poling, therefore, the spontaneous polarization of each domain must be reoriented approximately in the electric field direction against this stress, with the result that it takes 'a long time to effect the reorientati'on. Consequently, two requirements must be met in order to impart the largest possible piezoelectric characteristic to a ferroelectric material, i.e., poling by the application of a sufliciently high intensity DC electric field, and for a long time interval.

On the other hand, it must be remembered that all ferr'oelectric materials have a certain electric field intensity breakdown voltage, so that poling for an extremely long time interval at an impressed voltage less than said breakdown voltage is required. In other words, the conventional poling process is extremely disadvantageous in that in order to increase the piezoelectric characteristic to a satisfactory degree the proce-ss is too time-consuming and strict control of the impressed voltage required for the electric field intensity near the breakdown field intensity is necessary.

In order to eliminate these disadvantages, poling at high temperatures has been the common practice. It has been the belief that increasing the temperature would make thermal agitation vigorous and would also more easily reorient the spontaneous polarization of each domain, and furthermore that the higher the temperature, the more the structure would approach the cubic structure so that reorientation would be promoted.

The present inventors have conducted many experimental tests to obtain a comparison of piezoelectricity attainable by poling at high temperatures with that at room temperature. With these experiments, the electromechanical coupling factor in the radial mode (kr) was adopted as a measure of the assessment of piezoelectricity yas has been the common practice. Thus the larger the value of piezoelectricity, the larger became the value of kr.

Table 1 shows the experimental result obtained in this manner.

domain structure is gradually stabilized with a time elapse when the specimen is left standing in air and hence, the domain reorientation due to application of an electric TABLE i field becomes more diilicult to effect as time passes. Processing Value of 5 The present -inventors investigated the manner in which Composition tempera- 1:peroent the apparent spontaneous polarization (Ps. app), the aptme' C' parent residual polarization (Pr. app), and the apparent coercive eld (Ec. app.) vary with time after thermal dengs logfjtgfeffzfj:i: 100 y polarization by measuring hysteresis curves at room tem- 1gb (Zreszims) Oa+05 Wt. percent Gazon- (l) 13 l0 PCIEIUIC. PEIZTL;'gjg'ggpggggfgpgjjjjj @100 Table 2 indicates a result of our measurement of the Same as above 130 45 magnitudes of the above-mentioned characteristics appearing after a maximum AC voltage of 5 kv./mm. had *Room temperature. been applied for one minute.

TABLE 2 n Elapsed time Ps. app. Pr. app. Ee. app. Composition after thermal (IrC/cm!) (aC/cm?) (km/mm.) Remarks depolarization Pb (Zi'0.52'1io.is)Oa-l0.1 wt. percent Immediately after 25 21 1. 5

CliOa. depolarization. Same as above 14 8 1. 5 Same as above 12 6 0. 9 Propeller. Pb(Zr0.52Tin.4i)Oa-l-0.5 wt. percent 3 hours 13 11 1. 5

G3203. Same as above days 6 3 0.6 Do. Pb (Zrq.5i'li0.ri)Oe-l0.5 wt. percent Immediately after 27 26 1. 9

Feroe. depolarization. Same as above 1 day... 18 14 1. 5 Same as above 7 days.. 16 12 1. 2 Pb(Zr.51Tiu.ip)Os-l0.5 wt. percent Immediately after 24 22 2.0 f FeiOe. depolarization.

Same as above 1 day 17 11 1. 5

NoTE.-Propeller in the remarks column denotes a propeller type hysteresis loop depicted on the CRT screen.

The experimental results of Table 1 demonstrate that the higher the temperature in poling, the larger the value of kr and hence, the more excellent the piezoelectrical properties become.

Since piezoelectric materials inherently have semi-conductive properties, however, poling at high temperatures is attended with some disadvantages, as follows:

(a) A decrease in the electrical resistance, causing a current to liow in the material when a voltage is applied, which may render the poling process itself impracticable, and

(b) Rigorous temperature control requirements because the piezoelectric properties change with the processing temperature.

It has been generally considered that the degree to which the spontaneous polarization of each domain is reoriented in the direction of an electric iield in poling at room temperature or at high temperatures with a high intensity DC electric field applied is governed by three factors: the electric eld intensity during poling, the processing time, land the processing temperature.

Another important factor besides these three factors that cannot be overlooked is the stability of the domain structure prior to poling. Stability of the domain structure of a ferroelectric material is considerably affected by the carrier after the structure has been formed. This problem is dealt with in detail in a treatise entitled Polarization Changes During the Process of Aging in Ferroelectrics of the BaTiO3 Type by Z. P-ajak and I. Stankowski, which appears in Proceedings of the Physical Society, Dec. l, 1958, vol. 72, Pt. 6, No. 468, pp. 1144-1146.

The research by the authors of the above treatise indicates that the hysteresis loops obtained just after thermal depolarization are m-uch larger and rectangular in shape than those obtained after the specimen are left standing in air for many hou-rs lat the conclusion of the thermal depolarization process. This fact demonstrates that the Table 2 indicates clearly the tendency yof the values of Ps. app. and Pr. app. to be largest when measured immediately after thermal depolarization and to decrease gradually as the time elapse becomes longer. This experimental result demonstrates that the longer the elapse time after the thermal depolarization, the more diilcult reorientation of the domain becomes. f

Now a ferroelectric material which is heated above the Curie temperature loses its former domain structure, but a new domain structure will be formed after it is cooled and upon passing the Curie temperature. The fact that the new domain structure stabilizes with elapsed time is evident from the above-mentioned reference and our eX- perimental results shown in Table 2.

Such an increase in stability of the domain structure renders reorientation due to poling difficult and thus the effect of poling is diminished. These facts are clearly indicated in our experimental results shown in FIG. l.

FIG. 1 illustrates our experimental Iresult with a lead titanate zirconate lbase ceramic material consisting of a 52 mole percent lead zirconate, 48 mole percent lead titanate, and 0.5 wt. percent Fe203 as an additive agent and having the compositional formula:

The `curve in FIG. l illustrates the manner in which the piezoelectric characteristic expressed in the electromechanical coupling factor in the radial mode (kr) varies with the time interval between thermal depolarization and poling. Inspection of this curve readily reveals that the piezoelectric characteristic becomes reduced with an increasing time interval between thermal depolarization and poling.

It will be appreciated from the graph of FIG. 1 that stabilityof the domain structure is an important factor governing the poling effects and that if this is disregarded, piezoelectric materials of the saine quality could hardly be made on a predictable basis regardless of how rigorously the electric field intensity, processing time, and processing temperature for poling are controlled. This graph further suggests that poling undertaken immediately Further, according to the invention, a number of difficulties previously mentioned which attended the thermal depolarization process can be entirely obviated, while at the same time the advantage of poling immediately after after formation of the domain structure is extremely -addepolarization can be completely retained. vantageous in that it can be effected at `a relatively low The reasons for the improved piezoelectric properties electric field intensity, within a shorter time interval, and according to this invention may be understood from the at a comparatively lower temperature. following explanation. The pre-treatment at a high inten- A well known method of reorientation of the domain sity AC electric field causes rapid reversal of the direcstructure of a ferroelectric material consists of heating 10 tion of the electric field and changes the intensity of the it above the Curie temperature to destroy the old strucfield from time to time, and hence changes the direction ture and then decreasing the temperature below the Curie of spontaneous polarization of many domains in the matepoint thereby to form a new domain structure. This procrial. Thus, orientation of the domain structure that has ess is known as thermal depolarization. been gradually stabilized with a time elapse is agitated The manner in which the poling process effected imand a newly oriented structure appears. While the direcmediately after thermal depolarization is advantageous tion of spontaneous polarization of each domain is varied over the conventional poling methods effected for ferroby application of a high intensity AC electric field, electric materials after their domain structure has been stresses are created within the material, which remain fully stabilized wil-l be evident from the illustration of for some time even after removal of the AC electric FIG. 1. field to place the structure into an unstable condition. The carrying out of the thermal depolarization process Since the pretreatment helps to bring about an unstable prior to the poling process however, is difficult in quanstate both ferroelectrically land mechanically, succeeding tity production on an industrial scale for various reasons, -application of a high intensity DC electric field facilitates among which are: reorientation of the spontaneous polarization of each do- (a) Equipment for thermally depolarizing all ferromain in the direction of the electric field according to electric materials at once and in `large quantities must be the poling process. installed, To demonstrate the validity of this viewpoint, the pres- (b) All ferroelectric materials must be transferred to ent inventors investigated the manner in which the apthe poling equipment as rapidly as possible after the parent spontaneous polarization (Ps. app), the apparent thermal depolarization, residual polarization (Pr. app.) and the apparent coercive (c) Care must be exercised in poling so that the transfield (Ec. app.) vary with time after application of an fer may not be so hasty as to cause the processing tem- AC electric field regarding samples which had been left perature to vary or the internal temperature of the matestanding in air for many hours after the thermal depolarrials to become non-uniform and further, so that the difization. ferent products are not so markedly different in dimen- Table 3 indicates our experimental results with a lead sions as to result in significantly dissimilar cooling rates. titanate zirconate ceramic which can be expressed as:

To conclude, prior industrial poling methods which Pb(Zr0,52Ti0 48)O3l-0.5 wt. percent FezOa when a maxiresult in the desirable features of small variations of mum voltage -of5 kv./mm. was applied.

TABLE 3 Elapse of time after tlier- Time interval of Ps. app. Pr. app. Ee. app.

mal depolarization application of anAC (aC/cm!) (yC/em!) (kv./mm.)

electric field Immediately after depo- 1 minute 27 26 1.9

larization. Same as above.- 5 minutes 28 27 1. 9 Same as above.. 10 minutes 28 27 1. 9 1 day 1 minute 18 14 1. 5 Same as above 5 minutes 27 24 1. 8 Same as above l0 minutes 29 26 1. 9 1 Week 1 minute 16 12 1. 2 Same as above 5minutes 24 20 1.5 Same as above 10 minutes 28 26 1. 5

piezoelectric properties, reproducibility from product to product, and excellent piezoelectric properties of the products are attended with an impracticably high number of manufacturing difliculties.

Accordingly, it is an object of this invention to provide a new depolarization method. Another object of the invention is to harness for industrial purposes the advantages of poling prior to stabilization of the domain structure.

A further object is to utilize the experimental information illustrated in FIG. 1 by providing a new depolarization method whereby easy and rapid synchronization with the poling process can be achieved.

In accordance with an aspect of the invention, the above problems are obviated -by a poling process characterized in that an AC electric field in excess of the coercive electric field intensity and less than the breakdown field intensity of a ferroelectric material is -applied thereto for a suitable time interval prior to poling, and thereafter a DC electric field in excess of the coercive field intensity and less than the breakdown field intensity is applied to the material.

The experimental result shown in Table 3 indicates r that the values of Ps. app. and Pr. app. are decreased with increasing elapsed time after thermal depolarization and are increased with an increasing time period of application of a high intensity AC electric field to conform approximately to those of Ps. app. and Pr. app. just after the thermal depolarization if the application continues for l0 minutes.

FIGS. 2a, 2b and 2c illustrate the manner in which the hysteresis curve depicted on the cathode ray tube screen varies with time with an AC electric field applied to a sample of a lead titanate zirconate ceramic which can be expressed as:

Pb(ZI'0 52Tl0 4g)O3-l0.1 Wt. percent CI`203 had been left standing in air for one day after the thermal depolarization.

FIG. 2a denotes the configuration of the curve obtained when the AC electric field was applied for one minute, FIG. 2b that when it was applied for 5 minutes, and FIG. 2c that when it was applied for l0 minutes. The configuration of the curve thereafter remained substantially unchanged.

These experimental results clearly indicate the marked promotion of reorientation of the domain structure by the application of a high intensity AC electric field. The ceramic material having the compositional formula menobtained by varying the elapsed time after thermal depolarization, the conditions for application of a high AC electric field, and the conditions for application of a high DC electric field.

TABLE 4 'Time elapse after ther-mal Conditions for applying a high AC electric Conditions for applying a high DC electric Electromechanical coupling depolarization. field. field factor in radial mode (kr),

percent.

Immediately after depolari- Not applied Electric field at intensity 5.0 Irv/mm. 48

zation. appleid for 1 hr. at room temperature.

After one Week Not applied A Same as above 33 After one week... Electric field at 2.0 kv./mm. applied for Same as above 34 minutes at room tem erature.

ASame as above Electric field at 2.0 kv. mm. applied for Same as above 35 minutes at room temperature.

'Same as above Electric field at 2.5 liv/mm. applied for 5 Same as above 35 minutes at room temperature.

Same as above Electric field at 2.5 kvJnml. applied for 10 Same as above 47 minutes at room temperature.

Same as above Electric field at 3.0 kv./mm. applied for 5 Same as above 47 minutes at room tem erature.

Electric field at 2.5 kv. mm. applied for 10 Same as above 38 'minutes at 78 C.

.Same as above Not applied Electric field at 5.0 kv./mm. applied for 1 46 hr. at 100 C.

tioned immediately above was found to have an electrical resistance capable of withstanding a high intensity electric field at 100 C. FIG. 2d shows the configuration of the hysteresis curve measured at 100 C. It was proven by our experiment that changes in the configuration of the curve of FIG. 2d at 100 C. with elapsed time were less pronounced than changes in the configuration of the -curve at room temperature with elapsed time and maniested appreciably larger 'values of Ps. app. and Pr. app. from the beginning.

The experimental result of FIG. 2d is equivalent in :significance to the result shown in Table l which has proved that poling at high temperatures obtains more excellent piezoelectric properties than those obtained at room temperature. A comparison of FIGS. 2c and 2d further reveals that both configurations are quite similar and that the values of Ps. app. and P1'. app. are approximately of the same degree.

FIG. 3 shows similar results of our experiment as in the case of FIG. 2 with a lead titanate zirconate ferroelectric ceramic which can be expressed as:

The hysteresis curves in FIG. 3 indicate that the values 'of both Ps. app. and Pr. app. at room temperature increase with increased application time of an AC electric .field to the ceramis and that the hysteresis curve measured 'with an AC voltage applied for 10 minutes at room temperature is similar in the configuration to the curve measured with the same AC voltage applied for 10 minutes at `'an elevated temperature.

It will be seen that these various experimental results confirm the validity of the inventors theories. The inventors tested the effectiveness of the poling process according to this invention by cooling samples to 78 C. The poling process at such low temperatures was also effective. In other words, the poling process according to this insisting of a 52 mole percent lead zirconate, 48 mole percent lead titanate, and 0.5 wt. percent Fe2O3 as an additive agent and which can be expressed as:

Pb(Zro 52 Ti0.48)O3-l0.5 wt. percent Fe203, which were or more, and that (d) the value of kr can also be improved by application of an AC electric field even if the temperature is lowered to 78 C. Added for reference in the table is the result of poling by heating the material to 100 C. but the result of poling by applying a high intenstiy electric field is admittedly very comparable to the above-mentioned results.

While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be understood that the description is made only by way of example and not as a limitation of the scope of the invention as set forth in the objects thereof and n the accompanying claims.

What is claimed is:

1. A method of piezoelectrically activating a ferroelectrc material characterized by comprising successive steps o applying an AC electric field of an intensity in excess of the coercive electric field intensity and less than the breakdown field intensity of said material,

and applying a DC electric field of an intensity in excess of the order of the coercive electric field intensity and less than the breakdown electric field intensity of said material.

2. A poling method for piezoelectrically activating a fefrroelectric material which comprises the successive steps o exposing said material to an AC electric field having an intensity in excess of the coercive electric field intensity and less than the breakdown eld intensity of said material,

vention can find application for materials whose electrical and exposing said material to a DC electric fiield havresistivities are too low at room temperature, because the ing an intensity in excess of the order of the coercive electrical resistivities of such materials can be sufficiently electric field intensity and less than the breakdown increased by cooling to a low temperature adapted for the electric field intensity of said material.

poling. Therefore the poling process according to this invention may be said to be effective at any temperature References Cited unless the electrical resistance becomes too small to apply UNITED STATES PATENTS a high intensity electric field. L I

3,108,211 l0/l963 Alleman et al. B17-262 MILTON O. HIRSl-FIELD, Primary Examiner'.

J. A. SILVERMAN, Assistant Examiner. 

1. A METHOD OF PIEZOELECTRICALLY ACTIVATING A FERROELECTRIC MATERIAL CHARACTERIZED BY COMPRISING SUCCESSIVE STEPS OF APPLYING AN AC ELECTRIC FIELD OF AN INTENSITY IN EXCESS OF THE COERCIVE ELECTRIC FIELD INTENSITY AND LESS THAN THE BREAKDOWN FIELD INTENSITY OF SAID MATERIAL, AND APPLYING A DC ELECTRIC FIELD OF AN INTENSITY IN EXCESS OF THE ORDER OF THE COERCIVE ELECTRIC FIELD INTENSITY AND LESS THAN THE BREAKDOWN ELECTRIC FIELD INTENSITY OF SAID MATERIAL. 